Beat correction

Beat correction has two steps: find the bad intervals, then fix them.

Finding them. The common rule compares each interval with the ones around it. Kubios, for example, flags any interval further from a local average than a threshold: 0.45 s at its “very low” setting, then 0.35, 0.25, 0.15 and 0.05 s at “very strong” (Alcantara and colleagues, 2020). Lipponen and Tarvainen (2019) built an automatic method on the series of differences between successive intervals with a threshold that adapts to the recording; it caught about 97% of real ectopic beats ectopic beats Ectopic beat A heartbeat that starts outside the heart's normal pacemaker and usually comes early. Ectopic beats are common, but they distort HRV unless removed or corrected. More about Ectopic beat .

Fixing them. Peltola (2012) reviews the options: delete the bad interval, or replace it with the mean of its neighbours, a straight line between them (linear interpolation) or a smooth curve through them (cubic spline).

Why it matters for HRV.

• Deleting removes time from the series. Peltola reports that it was not recommended for RMSSD RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD and pNN50 pNN50 pNN50 The percentage of neighbouring NN intervals that differ by more than 50 milliseconds. It tracks fast, vagally driven changes, like RMSSD, but as a share rather than a size. More about pNN50 , may be acceptable for SDNN SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN , and that spectra should use interpolation instead.
• Even a few edits change short readings: under 5% of edited intervals can shift the results, and many studies drop a recording with under 80% normal beats. Kubios’s guide recommends keeping interpolated beats under 5%.
• Stricter settings edit real beats. Alcantara and colleagues found that SDNN differed by 39 ms on average between the very low and very strong settings in children, and that the very strong setting replaced 39% of the children’s intervals.

Plant a bad beat below and try each method.

Clean one bad beat

The teaching recording, 341 intervals

Gap before each beat, first 60 seconds Flagged

Clean recording Missed beat Extra beat Ectopic beat

Leave it in Delete Mean of neighbours Linear interpolation Cubic spline

Flagged: 1 of 341 intervals

Measure	Clean	Bad beat	Fixed
RMSSD (ms)	33.4	70.2	70.2
SDNN (ms)	28.7	51.7	51.7
pNN50 (%)	8.2	8.8	8.8
LF power (ms²)	120	1241	1241
HF power (ms²)	635	2314	2314

A missed beat makes one interval about twice as long as its neighbours. Left in, it lifts RMSSD from 33.4 to 70.2 ms, 2.1 times the true value, while SDNN moves less.

The buttons work when JavaScript is on.

A line of the gap between beats over the first 60 seconds of the teaching recording, with the planted artifact as a spike, a circle on each flagged interval and the corrected line drawn over it. A table gives RMSSD, SDNN, pNN50, LF power and HF power for the clean recording, with the artifact left in, and after correction, plus how many intervals were flagged. One group of buttons plants a missed beat, an extra beat or an ectopic beat, or restores the clean recording; another chooses the correction method: leave it in, delete, mean of neighbours, linear interpolation or cubic spline. A missed beat left in roughly doubles RMSSD; every method brings it back within a few percent.

The lesson How HRV is measured walks through the whole cleaning step.